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There are two vacuum ports on the 1.7L D-jet throttle body: one near the air bypass screw (idle speed screw), closest to the back of the car, and one on the opposite side, closests to the front of the car.

Then there's the vac unit on the distributor. There's a port pointing toward the distributor body, and one pointing away.

Can someone please tell which goes where, and a little theory of operation on the advance/retard functionality?

Also, when setting the timing at 27 degrees BTDC, are both vac lines removed from the distributor, and plugged?

When I got this car, it was only using one vac line, going from the port on the throttle body closest to the front of the car, connecting to the vac unit, on the port closest to the distributor body. To make things worse, that port on the vac unit has a torn diaphram, causing a vacuum leak.

I am trying to get the timing/ignition out of the equation, chasing a stumble during light load cruising or acceleration.

Thanks - Jim

The lower port on the TB (the front) goes to the lower port (the front) of the dist. This is the retard vacuum hose. The other is for the Vacuum advance. When setting the timing hoses off, plugged, to 27' @3500rpms. Then hook up the hoses should be set.
Geoff

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76 914 2.0L

Geoff gave you the correct routing for the hoses and ports.

The vac retard is active during closed throttle operation (idle, overrun) to reduce emissions (HC and NOx). The port is below the throttle plate.

The vac advance is active during light part-load (slightly cracked throttle, e.g. cruising conditions) to ignite the air-fuel mixture earlier, as it burns more slowly. The port is covered by the throttle plate when the throttle is closed, and is exposed when opened. When the throttle is only slightly open, the air velocity past the port is high due to the small exposed area, and significant vacuum is obtained. When the throttle is opened wide, the effective area is large and the air velocity at the port is low, and little vacuum is obtained.

The advance unit is dominant over the retard unit - if you pull a vacuum on both ports, you'll only get advance.

Later cars only had one of the two TB ports and used only one of the adv/ret units - I can never remember which one, and Dave Darling always has to remind me. I'm guessing it's the advance that isn't used?

Brad Anders

I've just learned that my interpretation of the vacuum advance is wrong, so here's the real poop.

Vacuum advance is active under part load over a wide range of engine speeds. At any particular level of part-load, manifold vacuum as a function of engine speed is a curve that rises to a maximum at some particular engine speed, then falls as engine speed approaches maximum. As a result, vacuum advance turns "on" at some threshold vacuum, increases to a maximum (mechanical stop in the dizzy), then decreases at high rpm when the manifold vac decreases.

It is NOT driven by the Bernoulli effect of air rushing by the port (like a Pitot tube), it's more of a direct measure of manifold vacuum. I'll post a good explanation of this at my website in the near future. In the mean time, make sure that the vac advance is disconnected when setting the mechanical advance setting.

Brad Anders http://members.rennlist.com/pbanders

So Brad, the advance side of things would have more of an effect on driveability, whereas the retard would have more of an effect on emissions? (I know that didn't sound like a question, but it was). The reason I am asking is that my vac unit, on the 205 distributor has broken diaphrams in both directions. I have another dual vac unit on it, but not the exact same. It doesn't always like to return to the same position, so I am planning on putting on single vac unit (with more of positive stop), as well as a better operating points plate. I just want to confirm whether the retard is more responsible for just emissions rather than performance.

Yes, the retard is for lower emissions (HC and NOx) at idle and during over-run. Note that the 914's TB is designed so that the air bleed screw can put the idle at spec level (about 950 rpm), assuming the vac retard is operating. If it isn't, it may not be possible to reduce the idle to spec levels. The 2.0 TB butterfly has a bleed hole in it, by blocking it, it may be possible to reduce the idle to spec levels even if the vac retard is inactive.

Brad Anders

Ok, so if you have all parts working (i.e. a working retard/advance vac unit), timing is set with both vac lines removed, and plugged. Afterwards, both vac lines are unplugged and replaced, in which the retard vac line will pull down the idle. Now set idle speed to spec with the air bleed screw. Then, have idle mixture checked/set; readjust idle (if needed) at air bypass. Am I missing something? Is this correct?

If you are having problems with your vac cannister and retard why not just get a rebuilt unit from FLAPS. Your distributor probably has a shot main shaft too so you can never get the timing spot on, (ie. the mark jumps around a lot with a timing lite).
This affects the FI points as well. Most of these cars have 30 year old distributor!
Yes, that is the way to set the timing and idle.
Geoff

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76 914 2.0L

F-friendly
L-local
A-auto
P-parts
S-store
Call and check around. Here in Seattle,the chain store sell the A1Cardone units for about $50. The ones I have seen have rebushed with bronze bushings. So look inside the box as the center shaft should not have ANY play or slop side to side. Pull a vacuum on each of the ports too to see it move the adv/retard plate.
Geoff

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76 914 2.0L

Damn, I feel kinda stupid (hehe). As much as I have been in the e-world, hadn't heard that one yet (FLAPS). Thanks. I'll give it a shot.

Ok. Could you give me more info on FLAPS (contact number, website, etc.)?

Thanks - Jim